The present invention concerns a lithium electrochemical generator comprising at least one composite electrode comprising an active material and a first and second solid electrolytes non-homogeneously distributed into the composite. The first solid electrolyte is mineral, vitreous or partly vitreous, is a specific conductor of lithium ions, and is preferably localized on the surface of the particles of active materials of the electrode. The second solid electrolyte is organic, comprises a dry or gelified polymer electrolyte that is a mixed conductor conducting ions surrounding the dispersed solid phases and acting as a deformable binder of the composite in contact also with the collector and the separator electrolyte of the generator. A thin layer of the first electrolyte wets and coats at least part of the surface of the active material particles to protect the coated surface of passivation or degradation reactions induced by the second electrolyte, and to maintain the quality of ionic and electronic exchanges between the active material of the electrode and the other components of the composite, the first electrolyte being impermeable to the components of the second electrolyte.
Polymer electrolytes are particularly desirable for the manufacturing of lithium accumulator using composite electrodes wherein the electrode materials are dispersed in the polymer matrix, that acts then as an ion conductor and as a deformable binder of the active matter powders. With such electrolytes, it is possible to control the elastomericity and the adhesion of the binding electrolyte in contact with the solids. These systems therefore allow the absorption, without any damage, of volume variations of the electrode materials.
Polymer electrolytes-based composite electrodes can be of two types, namely dry solvating type working at elevated temperatures, generally with lithium anodes; or gelified type, solvating or not, working at room temperature because of the addition of polar aprotic liquid solvents in association with electrodes of the lithium-ion type having cathodes working under high voltages (xcx9c4V). However, whatever their type, such composite electrodes age during cycling and/or with time. This ageing produces undesirable chemical or electrochemical reactions localized at the surface of the particles of active matter, at the surface of the collectors, and in certain instances, at the surface of the electronic conduction additive. This phenomenon, generic to lithium generators with organic electrolytes, causes the formation of passivation or degradation films on the surfaces where the ionic and electronic exchanges take place. The efficiency of the generators is therefore greatly impaired.
In polymedric media, these phenomena are amplified because in the solid state, the products formed by the reaction of the solvent, the salt and the electrode material have a tendency to accumulate because of the lack of convection, and are not compensated by the penetrating power of the liquid electrolytes that can maintain the exchanges at the interfaces because of their penetration power through the passivation or degradation films. The phenomena observed are electrochemical decompositions initiated by radicals, acid-base reactions, or oxidation-reduction reactions more or less catalysed by the materials present. FIGS. 1a) and 1b) illustrate the possible localization of passivation films on the various interfaces where the ionic and electronic exchanges take place.
It is therefore frequent in an organic solid medium to see an increase of transfer resistance at the interfaces, in addition to the limitation phenomena of ion diffusion at the electrodes. The films formed at the interfaces are sometimes detectable by electronic microscopy and can reach a few hundreds of nanometers. In other cases, the degradation reactions cause the breaking of the crystalline structure at the surface of the active phases, or cause the formation of soluble species harmful to the proper operation of the generator.
The reactions between crystalline or vitreous solids, for example between an oxide or a cathode chalcogenide and a vitreous electrolyte, are generally slower, and these systems are known for their stability versus time, temperature and operating voltage. The disadvantage of mineral solid electrolytes, crystalline or vitreous, is however their rigidity and fragility, which does not allow them to resist to electrode volume variations caused by cycling.
In accordance with the present invention, there is now provided an electrochemical generator, preferably a lithium electrochemical generator, comprising a cathode and an anode wherein at least one of them is a composite electrode comprising an active material, preferably in the form of dispersed particles, a current collector, and at least a first an a second solid electrolyte preferably non-homogeneously distributed in the composite, the first and the second electrolyte optionally comprising one or more dispersed electronic conduction additive. The first electrolyte comprises a mineral solid conductor, vitreous or partly vitreous, localized at the surface of the particles of the active material. The second electrolyte comprises an organic solid preferably comprising a dry or gelified polymer electrolyte which is a mixed conductor of ions surrounding the solid phases dispersed and acting as a deformable binder of the composite in contact also with the collector and the separator electrolyte of the generator. A thin layer of the first electrolyte wets and coats at least part of the surface of the particles of the active material, to protect the coated surface of the particles from passivation or degradation reactions, and to maintain the quality of ionic and electronic exchanges between the active material of the electrode and the other components of the composite. Further, the first electrolyte is impermeable to the components of the second electrolyte, and the second electrolyte is distributed in at least part of the composite to ensure the conductivity of the ions and acts as a deformable binder, preferably elastomeric, between the components of the composite electrode as well as with the current collector and the electrolyte separator of the generator.
In a second aspect of the present invention, there is provided a composite electrode comprising an active material, a current collector, a first mineral solid electrolyte and a second organic electrolyte, at least one of the first and the second electrolyte comprising at least one dispersed electronic conduction additive; the second electrolyte being in contact with the collector and acting as a deformable binder of the composite; wherein the first electrolyte coats at least part of the surface of the active material particles to protect the surface thereof from passivation or degradation reactions, and to maintain a quality of ionic and electronic exchanges between the active material and other components of the composite electrode, the first electrolyte being impermeable to the second electrolyte.
Finally, in a third aspect of the invention, there is provided a process for manufacturing an electrode according to the present invention, the process comprising the steps of:
a) mixing particles of electrode active material in an aqueous solution of the first mineral electrolyte comprising optionally a conduction additive;
b) drying the solution of step a) to obtain a powder of particles of the active material partially or completely coated with the first mineral electrolyte;
c) mixing the powder obtained in step b) with a second organic electrolyte optionally comprising a conduction additive; and
d) spreading the mixture obtained in c) on a current collector.
In a preferred embodiment, the first electrolyte is a lithium ion conductor. In a further preferred embodiment, the first electrolyte is vitreous or partially vitreous and conducts lithium or potassium alkaline ions. In a further preferred embodiment, the first electrolyte mainly comprises lithium polyphosphate of approximate formula (LiPO3)n wherein n greater than 3, and has a minimal ionic conductivity of 10xe2x88x9210 S/cm at the operating temperature.
In another preferred embodiment of the invention, the conduction additive comprises carbon black, graphites, metals like silver and copper and semi-metallic compounds comprising carbides, nitrides, borides, and suicides in a dispersed form, and mixtures thereof.
In another preferred embodiment, the second electrolyte comprises:
a solvating polymer, dry or gelified by a polar aprotic solvent rendered conductive by the addition of a soluble lithium salt or a polyelectrolyte; or
a gel comprising a low solvating polymer, a polar aprotic liquid solvent and a dissolved lithium salt.
The solvating polymer may be for example, a polymer electrolyte comprising a lithium salt and a homopolymer, a copolymer, a comb structure, or a network cross-linked or interpenetrated with a polyether. Preferably, the polymer electrolyte should adhere to the particles of the active material.
In a further preferred embodiment, the active material is used as the anode and comprises oxides, nitrides, carbon, graphites or mixtures thereof operating at a potential lower than 1.6 Volts versus a lithium metal electrode.
In a further preferred embodiment, the anode comprises lithium, a lithium alloy, a carbon or graphite insertion compound.